Electrolytic capacitor and electrolyte material therefor



United States Patent Alfred L. Jenny Pittsl'ield, Mam;

James 11. Curtis, Newark, NJ. 720.805

April 12, 1968 Dec. 15, 1970 General Electric Company a corporation of New York inventors Appl. No. Filed Patented Assignee ELECTROLYTIC CAPACITOR AND ELECTROLYTE MATERIAL THEREFOR 14 Claims, 2 Drawing Figs.

U.S. Cl 317/230, 252/622 Int. Cl. H0lg 9/00 Field 01 Search 317/230,

Primary ExaminerJames D. Kallam Attorneys-James J. Lichiello, Nathan J. Cornfeld, Melvin M.

Goldenberg, Frank L. Neuhauser and Oscar B, Waddell ABSTRACT: An electrolyte for electrolytic capacitors which includes as the conducting species thereof the reaction product salt of an organic base, and an acid such as a mono, di, or tribasic organic acid, the hydroxy-substituted derivatives of these acids or an inorganic acid. The concentration of the salt in the electrolyte can be from about 0.1 25 percent by weight. The electrolyte exhibits a low vapor pressure and enhanced chemical stability at elevated temperatures.

PATENTED DEC] 5 mm FIG.|.

INVENTORS: 1 JAMES H. CURTIS, ALFRED L. JENNY, I BY%W THE ATTORNEY.

ELECTROLYTIC CAPACITOR AND ELECTROLYTE MATERIAL THEREFOR BACKGROUND OF THE INVENTION l Field of thelnvention v The present invention relates to electrolytic capacitors, and more particularly to animproved electrolyte therefor.

2. Description of the Prior Art Conventional electrolytes for electrolytic capacitors for effective operation up to about 100C. generally contain a solv'ent, such as ethylene glycol, and a conducting species or ionogen. The ionogen employed is typically a borate such as ammonium pentaborateor ammonium biborate or to other substituted ammoniumsalts such as ammonium acetate and ammonium tartrate- For extended temperature devices, the

conventional glycol solvent may be replaced .by a highly polar, "nonaquwUS Solvent but the same conducting species is generally retained.

A major drawback of thescconventional electrolytes is their 'high vapor pressure at elevated temperatures, i.e., at about 125 C. This thermal instability result results in capacitors having unstable electrical properties. Additionally, the high vapor pressure increases leakage through the capacitorseals.

Whenutilized in electrolytic capacitors having aluminum. electrodes, the ammonia which is present. in the vapor pi'omotes chemical instability, pH reduction and resulting cor- -rosion of aluminum.

SUMMARY O THE I VENTION It is, therefore, anobject of the present invention to provide an electrolyte which exhibits a low vapor pressure and enhanced chemical stability at elevated temperatures.

It is another objectof this invention to provide an electrolytic capacitor having stable electrical properties at elevated temperatures and increased reliability by virtue of the chemical stability and low vapor pressure of the elecis a therrnally stable, conductive salt synthesized from an organic base and an organic or inorganic acid. The ionogen may be presentin .minimal but significant concentrations of from about 0.1 percent by weight to as much as. about 25.0 percent by weight. An embodiment of the present inventioncontemplatesan electrolytic capacitorcontaining this new electrolyte. [i

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from thefollowing description takenin conjunction with the accompanying.

drawings; FIG. I, illustratesa wound electrolytic capacitor section to which the present invention is applicablecand FIG. 2 illustrates a capacitor assembly with the capacitor section of FIG. I enclosed in a casing DESCR PTIO OETIIE PREFERRED EMBODIMENTS Referring now to the drawingsand inparticular to FlG..l, there is shown a partiallyunrolled exemplary capacitor roll section of conventional construction comprising a pair of electrode foils 1 and 2, at least one of which is made of tantalum, aluminum, or other suitable film-forming metal which may be advantageously, although not necessarilyetched, and which has a dielectric oxide film thereon, the foils in the operation of the capacitor having. opposite polarity. The dielectric oxide film may be. produced by film-,forming (usually anodizing)- processes ,;well -.known in the electrolytic capacitor art.

Between foils l and 2 are sheets of dielectric spacer material 3, 4, 5, and 6 composed of cellulosic material, including paper made of vegetable fiber, such as Benares Hemp, or other suitable material of permeable or porous impregnatable nature. After the foils and sheets are wound in the form of a roll section 7, these sheets are usually impregnated with the electrolyte of the present invention. Terminals or tapstraps 8 or 9 are secured to the respective electrode foils either before or during winding and extend in opposite directions from roll section 7. Rollsection 7 is then inserted into the casing 10, as shown in FIG. 2.

FIG. 2 shows the capacitor roll section 7'enclosed in a metal casingwith terminals Sand-9 extending respectively through insulating plugs or discs ll, 12 which provide a fluidtight seal with the interior of the casing. Roll 7 may be secured in the casing 10 by .a potting compound 14 in the bottom of 'casing 10. Compound 14 may be a resin which is nonreactive with and insoluble in electrolyte 15 of the present invention. Alternatively, other mechanical techniques well known in art may beemployed to stabilize the position of the roll 7 in casing 10. Electrolyte 15, as more fully described hereinafter, may fill casinglll.

In a polar .typecapacitor, only one of the electrode foils (anode) will have an anodic dielectric oxide film thereon. In a nonpolar capacitor both electrode foils will have such films thereon.

It should be understood that other types of electrolytic capacitors, employing an electrolyte such as is more fully described hereinafter, maybe used in accordance with the present invention. For example, instead of the arrangement shown in FIGS. .1 and 2, wherein roll section 7 is formed by wound cathode and anode foils, roll section 7 may include only one electrode foil (anode) having a ,dielectric film thereon, the cathode being the outer casing. In such an arrangement, the casing is typically made of silver. Another well known type of capacitor which may employ the electrolyte of Y the present invention is the wire capacitor, wherein the anode is a film-forming wire having a dielectric oxide formed thereon.

The electrolyte of thepresent invention is composed principally of a solvent in which is dissolved a relatively minor portion of an ionogen. The ionogen is a thermally stable, conductive salt synthesized from an organic base and an organic or inorganic acid and may be prepared by a neutralization reaction which can be complete, or partial as desired.

The organic bases suitable for use in the preparation of the ionogen of thepresent invention all have a characteristic in common, i.e., each contains a nitrogen atom, so that they may be broadly considered as substituted. ammonia compounds. Examples of organic bases include generally the alkanolamines, substituted alkanolamines, tetramethyl guanidine, piperidine and others.

The acids suitable for use with the present invention are ordinarily those acids whose. salts, resulting from reaction with an organic base, will support the formation of a dense oxide 1 plished by first mixing equal weights of base and water, and;

then adding acid until the pH reaches about 7.0 orless. The

mixture is then heated to about 13 C. to remove water, complete the reaction and stabilize the materiaLthermally..- When cool the form of the materials. vary from viscous liquid,-

glassy solid, semisolid or crystalline solid.

The electrolyte of the present invention may then be formed by adding the thus prepared conductive salt to a suitable solvent and dissolving it therein. At this point the mixture. if too alkaline, may be buffered by the addition of the acid component of the salt. If the electrolyte is to be used with aluminum electrodes, a desirable pH is in the range of about 5.5 to about 8. For added stability the entire electrolyte may be heated under reflux conditions at an elevated temperature to allow solvent/solute interaction, if any, to occur. A preferred method of preparing the electrolytes of the present invention consists of using the solvent as the reaction media and adding the acid and base thereto. The mixture is then heated for thermal stabilization and for reaction of the acid and base. By this technique, the electrolyte is prepared in a single step. It is particularly noteworthy that the electrolyte can be prepared in this manner because the neutralization reaction does not form water as a reaction product.

The solvents employed with the conducting salts to prepare the electrolyte of the present invention may be either aqueous or nonaqueous solvents but preferably should be polar materials. High polar solvents usually have higher dielectric constants. and greater dissociation with ultimately better solubility of salt therein. Nonpolar solvents, for v example carbon tetrachloride, would be substantially less effective in dissolving the conductive salts described herein. In the event that the conductive salt is insoluble, or soluble only with difficulty in the chosen solvent, it may be dissolved in a mutually compatible second solvent and then incorporated to form the electrolyte. Examples of solvents which may be used to dissolve the conductive salts are water, ethylene glycol, dimethylformamide, dimethylacetamide, dimethyl or diethyl cyanamide, butyrolactone, n-methyl pyrrolidone, and others, as well as mixtures of any of these solvents. The type of solvent is chosen generally to satisfy the voltage and temperature conditions required by the particular applications of the capacitor.

The conductive salt as described is particularly useful when added in concentrations of from about 0.1 percent to about 25 percent by weight. The preferred concentration range depends upon the particular capacitor application but generally will be in the-range of from 4 percent to l percent by weight.

The resultingiel'ect'rolyte containing the conductive species, 1 water, is a substantially nonaqueous liquid. in such a case the only water which is pres'ent would be derived from the water of crystallization of the ionogen. As a result the usual problems associated with Al-water reaction at the cathode and hydration f the anodic film are eliminated. ln another-embodiment, although not preferred, the electrolyte could advantageously'be a thermally reversible gel.

On the basis of the stoichiometry of the reactions, the following set of equations may be employed to define the reaction and the conductive salt reaction products:

' cording to equation (I). Theoretically the equation would predict that 49.2 grams of acid would be required. An actual experimentation utilized 48.5 grams of acid. The difference is well within the experimental error of the synthesis.

Applying equation (l) to the monoethyanolamine-acetic acid reaction. it is'noted that Rewriting equation ('l) in terms of this example to determine the formula for the conducting salt.

A similar test was conducted to establish the validity of equation (2). Forty (40) grams of triethanolamine were reacted with adipic acid. The equation theoretically predicts that 20 grams of the acid will be required, while 22 grams were actually used in an experimentation. Again this difference on correspondence is within the experimental error of the synthe- SIS.

Applying equation (2) to the triethanolamine-adipic acid reaction wherein The-formula for the resulting conducting salt can be deter- The salts formed are simple adducts and unlike the conventional neutralization reaction, no water is formed. It is this characteristic which permits the solvent to function as a desirable reaction media for the acid and base.

The conductive salts on dissolution in an appropriate solvent confer substantially improved properties thereon relative to conventionally employed electrolytes. Foremost among these em improvements is the resulting lower vapor pressure at elevated temperatures. For example, at 125 C., electrolytes of the present invention containing a conductive salt as herein before described exhibit vapor pressures from about 100 to 200 mm. Hg less than electrolytescontaining conventional ammonium salt in ionogens. Other significant improvments include enhanced chemical stability at elevated temperatures, controlled water content for maximum scintillation voltage, minimum reactivity with aluminum Al and A1 0 and inhibition of corrosion of Al and degradation of A1 0 through the use of a buffering system to maintain pH.

In order to demonstrate the effectiveness of the electrolytes of the present invention for use in electrolytic capacitors nuv EXAMPLE] The effect of the conductive salt composition and concentration on the 25 C. resistivity of a solvent comprising a mix ture of 75 percent by volume dimethylformamide and 25 percent by volume ethylene glycol is shown in table I for a wide range of conductive salts. Capacitors utilizing these electrolytes are generally operable in the temperature range from about 55 C. to 125 C.

EXAMPLE ll The effect of the conductive salt composition and concentration on the 25 C. resistivity of a solvent comprising a mixture of 100 parts ethylene glycol and 3 parts water is shown in table". The resistivities are noted for conductive salt concentrations of 5 grams and 10 grams respectively per 103 grams of solvent. Capacitors utilizing these electrolytes are generally useful in the temperature range from about 20 C. to 85 C.

TABLE I Concentration of grams or iiii R istivlt r es y W so! ohm-em: Salt ofsolvent at 25"v C.

' Monoethanolamine and malonlc acid. i 6 561 8 524 Concen- Concentration tration of grams 01 grams of suit oi salt per 100 Resistivity, 5 per 100 Reslsnvlty. grams of ohm-cm. grams of ohm-cm. Salt of--- solvent at 25 C. salt F o e a 2 C- Monoeihanoiamine and succinic acid i 3W5? T 3 ,92g 6 179 nethanolamlne and bone and. 6 g 587 Monocthanoinmine and artaric cid 2 1. 42 Monoethanolamine and adipic acid .2 1, 663' Monoethanolamine (mic acid 2 716 I5 N-amiuoeth ylethanoiamme and succinic acid. 1 2 3, 542 4 248 6 9 007 6 1, 07 6 Dlethanolamme and mane acid 2 1' 529 N, N t y t ano a un an 8 11110 acl i 113 g 6 924 8 743 8 785 methanolamine and i i acid i 1,1 13 N methyldiethanolamme d acid" 22g 6 8 3,388 8 555 Dicthanolamine and adipic acid- 2 2, 495 Diethanoi'aminc and citric acid 2 2, 14 3 4 462 25 a 1,133 v 6 260 Dlethanolamlna and tartaric acid 2 1, 742 N N dimethylethamlamlie and f 801d" '7' i g 33% c ,585 s 865 8 508 Dietha flamjne and citric acid; 2 2, 146 Triethamlamme adplc i g a; 4 462 6 2' 264 N methyl diethanolamino and oxalic acid.-.. 2 757 Nfinethyl dlethanolamme and amp: acid i 22 g 6 1, 202 W 8 871 I r 8 1,066 Ndnethyl methanohmme and tarmac acid n 2 1. 648 N-methyl diethanolamme and succmlc ac1d I i 1, VI

N, N dimethylethanolamine and malic acid.. 2 229 Triethanolamine and tartaric acid i 40 2, a 1:135 8 413 3 995 N-aminoethyl ethanolaminc and adipic acid. i g, 3%: I I 6 21469 Tricthanoiamine and citric acid 2 2, 43 I 8 I, N-methyi diethanoiamine and citric acid- 2 1,120 4 8 11051 g "328 N-mcthyi diethanoiamine and malonic acid..." 2 1,602 v I 8 g N -methyl diethanolamiue and propionic acii-v. 2 2, 288

i v 4 1,428 a 038 1 6 1,101 Diethanolamine and oxalic acid Z 1, 1 8 946 11 N-aminoethyl ethanolamine and propionic a r I g acid.. 2 1,823 v 9 4 1, 12s Diethanoiamine and malonic acid 2 $3??? 55 g 5 1, 435 N-aminoethyl ethanolamine and acetic acid- 2 1 342 s 1, 292 4 Dimcthyl cthanolamine and succinic acid..,.. 2 I V g 22 6 468 I v v 8 407 Dicthanoiaciine and propionic acid..A i 2a Dimethy] ethanolamine and tartaric acid g 1, 156 g 59s I 3 N-aminoethyl ctbanolaminc and malonic acid- 2 1, 968 Dimcthyl ethanoiamine and oka'iic acid .Q.-. 2 1, 008 g 4 672 8 3 o 546 1 V 478 Tetramethylguanidine and phosphoric acid.... 2 1, 081 Triethanolamine and phosphoric'acid..; 2 3,111 4 688 4 Tetiamethylguanidine and succinic acid 2 720 6 1, 802 4 410 8 1, 602

' Tetrameth i anidi c d t it cid N-mcthyidieihanoiamine and phosphoric acid. i, y t u an a me a i :6 V 509 Tetramethylguanidiae and citric acid. .i 2 s62 4 Diethanoiaminc and phosphoric acid 2 3,172 511 4 2,325 Tetramethyiguanidine and malic acid 2 702 6 1, 940 4 421 Concentration I of, vzzrams or sdn I per 100 Resistivity, t 7 grams of ohm-cm; Salt ol' solvent at 25? lipt'litlillt and adipic acid... 1,133 I i 4 733 6' 585. I I s 50a Tctramcthylguanidine and adipic acid r Monoisopropanolamine and citric acid 1, y

'Monoisopropanolamineand adipic acid i v i 1 1,

Pipcridine and tartaric acid"... 2 1, 152 v I 4 752 I 6 601 Piperldinc and oxalic acid -2 i 1, 359 v 4 s9 6 734 8 644 Piperidinc and succinic acid 2 702 4 3 Monoisopropanolamine and succinic acid. :..1. 3 i 1 323g 6 557 8 505 TABLE II Ohms/cm. resistivity I for-- 1- Salt of. grams 10 grams Monoethanolamine, formic acid 2,019 1, 215 Tricthanolamine, formic acid. 1, 292 733 N-aminocthyl ethanolarnine, form a 4, 264 2, 423 'N-methyldicthanolamine, formic acid 843 497 -I)iethanolamine, acetic acid 1, 615 988 N-methyi diethanolaminc, acetic aci 1.163 698 Monoethanolamine, propionic acid.. 1, 744 1, 040 'Diethanolarnine, propionic acid 2, 267 1, 417 N-aminocthyl cthanolamine, propionic acid 2,196 1, 350 N -methy diethanolamine, pr opionic acid 1, 481 876 'Iricthanolamine, oxalic acid... 1, 512 1, 344 NN-dimethyl ethanolamine, oxalioacid. 1,005 589 Monocthanolamine,maionic acid;;... 1, 014 580 N-aminocthyl ethanolamine, malonic aci 3, 617 2, 786

N-methyl diethauolamine, malonic acid 1, 520 924 NN-dimethyl ethanolamine, succinic acid 1, 082 727 N -aminoethyl ethanolamine, succinic acid 4, 025 2, 383 N -methyl. diethanoiamine, succinic acid 1, 421 875 Tet-ramethyl guanidine, succinic acid ,063 620 Monocthanclamine, malic acid 1, 734 1,032 'Dicthanolamine, malic acid 1, 576 I 985 .Triethanolamine,-malic acid 1, 809 1, 221 NN-dimethyi ethanolamine, malic acid 1, 143 701 N-mcthyl dicthanolamine, malic acid... 1, 309 888 Monoethanolamine, tartaric acid 1, 244 814 NN-dimethyl ethanolamine, tartaric acid 729 N -methyl diethanolamine, tartaric acid... 1, 880 1, 124 Piperidine,vtartaric acid 1, 404 819 :Tctramethyl guanidine, tartaric acid 1, 439 840 N N-dimethyi ethanoiamine, citric acid 1, 664 .N methyl diethanolaminc, citric acid 2, 270 1, 445 Piperidineycitric acid 1, 510 898 'Tctramethyi guanidine, citric acid 1, 483 853 Monoisopropanolamine, citric acid 1,088 660 'ltiouoethanolamine, phosphoric acid 1. 502 969 'Diethanolamine, phosphoric acid. 2, 460 1, 617 Tricthanolamine, phosphoric acid 3,011 N-methyi diethanolamine, phosphoric acid... 2, 270 1, 386 Tctramcthyl guanidine,.phosphoric acid 1 1, 546 895 Monoisopropanolamine, phosphoric acid 1, 350 833 I Monoethai' olamine, adipic acid 1. 835 1,030 Diethanolaminc, adipic acid.... 2, 455 1, 583 'lriethanolamine, adi ic acid. 3, 028 NN-dimcthylethano amine, adipic acid 1, 483 912 N-aminocthylcthanoiaminc, adipic acid 4, 974 2, 972 N-mcthyl diethanolamine, adipic acid 1, 544 I 946 Piperi ine, adipic acid 1, 152 t 681 Tetramcthyl guanidine, adipic acid... 1, 586 924 Monoisopropanolamine, adipic acid ,060 626 Pipcridine, lactic acid 1, 098 652 1 No data.

EXAMPLE ill Twenty grams of'mondethyanolamine were reacted with 1.4.3 grams ofinitric acid, Water was utilized-as a'mutual' cm. at C. A 4 percent solution has.a resistivity of 1'88vohmi I XA YWL W Twenty (20) grams of triethanOIamine-"w'ere reacted wit'h 8.4 grams of nitric acid to form the conductive nitra teZ' A 4 percent by weight solution of the nitrate in dimethylforrnami'de has a resistivity of 25 ohm-cm.

lvent, this conductive salt gave the following re- Percent by weight conductiv 'salt i' Id a 'diriiethylforr namide solvent, thefollowing resistivities:

"Resistivity,

ohm-cm.

EXAMPLE v1 Seventeen and one-half 17.5) grams monoisopropanolamine were reacted with 18 grams of nitric acid to form the conductive nitrate. In a dimethylformamide solvent, the following resistivities were noted:

Resistivity, ohm-cm. 273

Percent by weight conductive salt:

EXAMPLE vn Resistivity, Percent by weight conductive salt: ggz

it is especially noteworthy that the pH of nitrate {close to 7.0. When using nonaqueous solvents the pH- must be kept as close to' 7.0 as possible and the water content must be kept low to prevent degradation of the electrolyte by hydrolysolvent iortlie reactants and excess-water was removed, by" heating. Anv lectrolyte consisting of 2 percentl'by weight-of the reactionpro duct conductivenitrate salt and 98 percerit by I weight din cthylformamide exhibited a reS istivity.ofi285ohm-t Resistivity,

the" conductive salt gave derstood that numerous modifications can be made by those the invention.

tration of the conducting adduct is from about 4 to about 10 particular N-methyl ethanolamine and Y skilled in the art without actually departing from the scope of IO We claim: v

I. An electrolyte for electrolytic capacitors essentially cornprising a nonaqueous solvent and a conducting adduet dissolved therein. said adduet consisting or the reaction product of a nitrogen-containing organic base and an acid selected from the group consisting of di and tri carboxylic organic acids. 2. An electrolyte as claimed in claim I wherein saidacid is selected from the group consisting of adipieand succinic acids.

3. An electrolyte as claimed inclaim I wherein said nitrogen-containing organic base is selected from the'group consisting of the alkanolamines. substituted alkanolamines. tetramethyl guanidine and piperidine.

4. An electrolyte as claimed in claim 3 wherein said nitrogen-containing organic base is selected from the group consisting of the alkanolamines and the substituted alltanolamincs.

1 ing of the alkanolamines and substituted .5. An electrolyte to claim 4. said nitrogen-containing organic base being selected from the group consisting of N-methyl ethanolamine and methsnolsrnine. I

6. An electrolyte as claimed in 5 wherein the concertpercent by weight.

7. An electrolyte as claimed in claim 1 wherein the concentration of the conducting adduct is from about 0.l to about 25 percent by weight.

8. An electrolytic capacitor comprising a pair of electrodes. at least one of said electrodes being composed of a film-forming metal having a dielectric film thereon. and an electrolyte comprising a nonaqucous solvent and a conducting adduet dissolved therein, said adduet consisting of the reaction product of a nitrogen-containing organic base and an acid selected from the group consisting of di and tri carboxylic organic acids. and being capable of supporting the formation of said dielectric film.

9. An electrolytiecapacitor as claimed in claim 8, wherein said acid is selected from the group consisting of adipic and succinic acids.

10. An electrolytic capacitor as claimed in claim 8, wherein said nitrogen-containing organic base is selected from the group consisting of the alkanolamines. substituted alkanolamines, tetramethyl guanadine and piperidine.

II. An electrolytic capacitor as claimed in claim 10. wherein said organic base is selected from the group consistalkanolamines.

12. An electrolytic capacitor, as claimed in claim 8, wherein the concentration of the conducting adduct is from about (H to about 25 percent by weight.

13. An electrolytic capacitor, as claimed in claim 12, wherein the concentration of the conducting adduct is from taining an adduet selected from the group alkanolamine I adipate and alkanolamine succinate. 

